Reference-point return method

ABSTRACT

The invention relates to a reference-point return method for returning a movable element of a machine to a reference point using solely a linear scale, without relying upon a limit switch. The method includes providing the linear scale (1) with a second scale portion (2) for stipulating a deceleration starting position and a reference-point position, in addition to first scale portion (3, 4) for generating two-phase signals for position detection, generating signals (S 1 , S 2 ) indicative of the deceleration starting position (D) and reference-point position (O) from the second scale portion (2) of the linear scale, slowing a reference point return rapid-traverse velocity (Va) to a reference-point return rapid-traverse velocity (Vb) using the deceleration starting signal (S 1 ), and effecting return to the reference point by stopping movement in response to the reference-point signal (S 2 ).

DESCRIPTION

1. Technical Field

This invention relates to a reference-point return method and, moreparticularly, to a reference-point return method for returning a machinetool table or the like to a predetermined reference point by an outputsignal from a linear scale.

2. Background of the Invention

A reference-point return method is available for moving a machine toolin the direction of a predetermined reference point, reducing thereference-point return velocity in response to generation of adeceleration signal, and stopping a movable element in response to aninitial one-revolution signal, which is indicative of one revolution ofa motor, following the generation of a signal indicating the proximityof the reference point. FIG. 5 is a view for describing such areference-point return method. Here a movable element (table) TB of amachine is provided with a limit switch LS, and a stationary portion MCof the machine is provided with a reference-point return dog DG in thevicinity of the machine reference point. If the reference-point returnmode is established when the limit switch LW resides in a working areaAIII on the left side of the dog DG, the movable element TB of themachine is moved toward the reference point at a rapid-traverse velocityV_(H). When the vicinity of the reference point is reached and the limitswitch LS contacts the reference-point return dog DG at time t₁, a limitsignal LSS makes a transition from "1" (high level) to "0" (low level),as shown in FIG. 5. The reference point return velocity is reduced inresponse to the negative-going transition (deceleration signal) of thelimit signal LSS. At time t₂, the limit switch LS parts from the DG andthe limit signal LSS reverts from "0" to "1". The traveling velocity atthis time is V_(L). When the limit signal LSS reverts from "0" to "1"(rises), the movable element TB of the machine subsequently moves towardthe reference point at the velocity V_(L) and stops in response togeneration of an initial one-revolution signal ORS, which is indicativeof one-revolution of a motor, following the occurrence of thepositive-going transition (reference-point proximity signal) of thelimit signal LSS. OTA in FIG. 5 represents an over-travel area, andfirst, second and third areas AI, AII and AIII represent an area on theover-travel side, a reference-point proximity area, and a working area,respectively.

In order to stipulate a predetermined position such as adeceleration-start position in the prior art, a mechanical switch suchas the limit switch LS switched at the predetermined position isrequired. By relying solely upon the signal from this switch, theaccuracy of the reference-point return operation is greatly influencedby the mounting precision and operating precision of the switch.Accordingly, the prior art is such that reference-point return controlis performed by making joint use of this signal and the motorone-revolution signal generated by a linear scale or rotary encoderoriginally mounted on an NC machine tool.

With the prior art, however, the fact that the limit switch is usedraises cost and entails a complicated mounting operation.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide areference-point return method in which signals indicative of adeceleration starting position and a reference-point position aregenerated solely by a linear scale, and without using a mechanicalswitch, to return a movable machine element to the reference point.

The present invention relates to a reference-point return method forreturning a movable element of a machine to a reference point usingsolely a linear scale, without relying upon a limit switch. In thepresent invention, the arrangement is such that a linear scale generatessignals indicative of a deceleration starting position and areference-point position. Reference-point return velocity is reducedusing the signal indicating the deceleration starting position, andmovement of the movable machine element is stopped in response to thesignal indicative of the reference-point position. Return to thereference point is thus achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for describing the general features of the presentinvention;

FIG. 2 is a perspective view of the operating principle and shows theconstruction of a linear scale;

FIG. 3 is a control block diagram of a machine tool capable ofpracticing the reference-point return method of the present invention;

FIG. 4 is a flowchart of processing according to the present invention;and

FIG. 5 is a view for describing the reference-point return methodaccording to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a view for describing the general features of the presentinvention, and FIG. 2 is a perspective view showing the construction ofa linear scale.

With reference to the drawings, numeral 1 denotes a linear scale havinga position scale portion 2 for stipulating an acceleration startingposition and a reference-point position, and position measurementportions 3, 4 which generate A-phase and B-phase signals for detectingposition. Va represents a rapid-traverse velocity for return to areference point, Vb a velocity for return to the reference point, S₁ adeceleration-start signal, and S₂ a reference-point position signal.

The linear scale 1 comprises a glass-like transparent plate, and masks(the shaded portions in the drawings) for blocking the transmission oflight through the glass-like transparent plate. A deceleration startingposition D and a reference-point position O are set at respective endsof the mask of the position scale 2. The two-phase signals A, B formeasuring the position of the movable machine element TB shown in FIG. 5are generated by virtue of the mask patterns of the position measurementscale portions 3 and 4. The two-phase signals A and B are 90° out ofphase.

FIG. 2 is a perspective view of the operating principle and shows theconstruction of a linear scale. A light-emitting element 11 and alight-receiving element 12 are arranged to oppose each other with thelinear scale 1 and a scale 13 on the light-receiving side disposedtherebetween. These are arranged to detect the transmission of lightthrough the linear scale. The signals S₁, S₂ indicative of thedeceleration starting position D and reference-point position O aregenerated by light which has passed through the position scale portion2. Though only one light-emitting element and only one light-receivingelement are shown, in actuality these are provided for each of the scaleportions 2 through 4 and they are so arranged that the transmitted lightbeams will not interfere with one another. The linear scale 1 is securedto the stationary portion MC of the machine shown in FIG. 5, and thelight-emitting element 11, light-receiving element 12 and scale 13 onthe light-receiving side are provided on the movable element TB of themachine to move in unison therewith.

FIG. 3 is a control block of a machine tool capable of practicing thereference-point return method of the present invention.

Numeral 21 denotes a processor which exercises overall control, 22 a ROMstoring a reference-point return control program according to thepresent invention, and 23 a table drive unit for rotating the motor M todrive the movable machine element TB via a ball screw BS coupled to therotary shaft of the motor M. Numeral 24 denotes an interface unit forinputting the deceleration starting signal S₁ and reference-pointposition signal S₂ to the processor 21 using a position signal PSobtained from the light-emitting element 11 and light-receiving element12, by way of example. The processor 21 performs control to effectreturn to the reference point by slowing the reference-pointrapid-traverse velocity Va to the reference-point return velocity Vbusing the deceleration starting signal S₁, and stopping the movableelement by the reference-point position signal S₂.

FIG. 4 is a flowchart of reference-point return processing according tothe present invention. Reference-point return control of the presentinvention will now be described with reference to FIGS. 1 through 4.

First, when a button on an operator's panel, not shown, is manipulatedto command a reference-point return mode, the processor 21 is placedunder the control of the reference-point return control program storedin the ROM 22 and executes reference-point return control, describedbelow. Specifically, the processor 21 causes the movable machine elementTB to move toward the reference point at the reference-pointrapid-traverse velocity Va (step 101). When the vicinity of thereference point is reached at time t₁, the position PS outputted by thelinear scale 1 makes a transition from "1" (high level) to "0" (lowlevel), as shown in FIG. 1. In response to this negative-goingtransition of the position signal, the interface unit 24 generates thedeceleration starting signal S₁ (step 102). The processor 21 responds tothe deceleration starting signal S₁ by slowing the traveling velocity tothe reference-point return velocity Vb (step 103). The position signalPS outputted by the linear scale 1 reverts from "0" to "1" at time t₂.When the position signal makes this transition from "0" to " 1" (i.e.,when the signal rises), the interface unit 24 generates thereference-point position signal S₂ (step 104). As a result, theprocessor 21 recognizes the position at which the reference pointposition signal was generated as being the reference-point position,stops the rotation of the motor M and ends reference-point returncontrol (step 105).

It should be noted that the position signal PS outputted by the linearscale 1 can be replaced by one obtained by inverting this signal level,as indicated by the dashed line. The reference-point position can beshifted if this arrangement is adopted.

Further, it is possible to set a numerical value N in advance, generatethe reference-point position signal S₂ and subsequently stop the movablemachine element, with the position at which N-number of the two-phasesignals have been generated following generation of the signal S₂ beingtaken as the reference point.

Thus, in accordance with the present invention, the arrangement is suchthat a linear scale generates signals indicative of a decelerationstarting position and a reference-point position, reference-point returnvelocity is slowed using the signal indicating the deceleration startingposition, and movement is stopped in response to the signal indicativeof the reference-point position, thereby achieving return to thereference point. As a result, reference-point return can be performed inhighly accurate fashion without using a mechanical switch.

We claim:
 1. A reference-point return method for returning a movableelement of a machine to a reference point using a linear scale having afirst scale portion and a second scale portion comprising the stepsof:(a) obtaining a deceleration starting position using the second scaleportion; (b) obtaining a reference-point position using the second scaleportion; (c) generating signals indicative of the deceleration startingposition and the reference-point position using the second scale portionof said linear scale; (d) slowing return velocity using the decelerationstarting signal; and (e) returning the movable element to the referencepoint by stopping movement of the movable element in response to thereference-point position signal.
 2. A reference-point return methodaccording to claim 1, wherein said linear scale includes a glass-liketransparent plate having masks, and wherein step (c) includes thesub-step of:blocking transmission of light by using the masks on theglass-like transparent glass.
 3. A reference-point return methodaccording to claim 2, further comprising the step of:producing the maskson the glass-like transparent plate so as to vary the light transmissionat the deceleration starting position and at the reference-pointposition.
 4. A reference-point return method according to claim 1,further comprising the steps of:(f) generating two phase signals fromthe first scale portion; and (g) stopping movement of the movableelement when a predetermined number of the two-phase signals have beengenerated after generated of the reference-point position signal.